Variation Of Laser Intensity Research Articles

Formation of electromagnetically induced transparency and two-photon absorption in close and open multi-level ladder systems

We report the observation of electromagnetically induced transparency (EIT) and two-photon absorption (TPA) in rubidium vapor cell kept at room temperature. The experiment is performed under multi-level ladder configuration acted upon by a weak probe laser and a strong control laser. The control laser couples the 5P3∕2→5D5∕2 transition such that we observe EIT as the probe laser scans the 85Rb 5S1∕2F=3→5P3∕2F′ transitions whereas two-photon absorption is observed as the probe laser scans 85Rb 5S1∕2F=2→5P3∕2F′ transitions with an increased probe power. Furthermore, both EIT and TPA peaks show shift in peak positions with variation in the control laser intensity. The experimental findings are compared with the simulated probe response based on the theoretical model of a multi-level ladder (Ξ) type system. Occurrence of EIT and TPA are also explained on the basis of dressed state picture.

Optimal control of Raman pulse sequences for atom interferometry

We present the theoretical design and experimental implementation of mirror and beamsplitter pulses that improve the fidelity of atom interferometry and increase its tolerance of systematic inhomogeneities. These pulses are designed using the GRAPE optimal control algorithm and demonstrated experimentally with a cold thermal sample of 85Rb atoms. We first show a stimulated Raman inversion pulse design that achieves a ground hyperfine state transfer efficiency of 99.8(3)%, compared with a conventional π pulse efficiency of 75(3)%. This inversion pulse is robust to variations in laser intensity and detuning, maintaining a transfer efficiency of 90% at detunings for which the π pulse fidelity is below 20%, and is thus suitable for large momentum transfer interferometers using thermal atoms or operating in non-ideal environments. We then extend our optimization to all components of a Mach–Zehnder atom interferometer sequence and show that with a highly inhomogeneous atomic sample the fringe visibility is increased threefold over that using conventional π and π/2 pulses.

Circular ripple patterns on silicon induced by bubble-diffracted femtosecond laser pulses in liquid.

We report on a new technique of silicon surface nanostructuring in liquid with a pair of Gaussian-shaped femtosecond laser pulses. The bubble, generated in liquid near the molten silicon surface by the first pulse, serves as a dynamic microscale obstacle for spatial modulation of the intensity profile of the second pulse following at a certain delay via scattering processes. As a result, the circular ripple patterns with anomalously high surface-relief modulation, undersurface annular nanocavities, and interfacial smoothness are produced at the surface. The possibility of the control over the specific pattern through the laser intensity variation is shown.

2D-Infrared Spectroscopy of Proteins in Water: Using the Solvent Thermal Response as an Internal Standard.

Ultrafast two-dimensional infrared (2D-IR) spectra can now be obtained in a matter of seconds, opening up the possibility of high-throughput screening applications of relevance to the biomedical and pharmaceutical sectors. Determining quantitative information from 2D-IR spectra recorded on different samples and different instruments is however made difficult by variations in beam alignment, laser intensity, and sample conditions. Recently, we demonstrated that 2D-IR spectroscopy of the protein amide I band can be performed in aqueous (H2O) rather than deuterated (D2O) solvents, and we now report a method that uses the magnitude of the associated thermal response of H2O as an internal normalization standard for 2D-IR spectra. Using the water response, which is temporally separated from the protein signal, to normalize the spectra allows significant reduction of the impact of measurement-to-measurement fluctuations on the data. We demonstrate that this normalization method enables creation of calibration curves for measurement of absolute protein concentrations and facilitates reproducible difference spectroscopy methodologies. These advances make significant progress toward the robust data handling strategies that will be essential for the realization of automated spectral analysis tools for large scale 2D-IR screening studies of protein-containing solutions and biofluids.

Enhanced Hybrid Copper Conductive Ink for Low Power Selective Laser Sintering

Copper conductive ink has great potential as an electrode material for flexible electronics due to its low-cost relative to silver conductive ink while having comparable electrical conductivity. The process of fabricating anti-oxidation hybrid copper conductive ink, however, results in an increase of required sintering energy; a high sintering temperature is disadvantageous due to the possibility of thermally damaging the substrate. In this study, selective laser sintering of new hybrid ink compositions was investigated by varying laser intensities and comparatively analyzing the resulting electrical conductivity. Then, the thermal behavior of fabricated inks during the sintering process was studied using experimental and numerical approaches. A finite element model was developed to simulate the laser heat flux and irradiation paths on the ink. Finally, the obtained results of the thermal profile were verified experimentally.

Optical surface measurements of a wavelength-tuned Fizeau interferometer based on an optical power, real-time feedback and compensation system

The wavelength-tuned Fizeau interferometer achieves the desired phase for interferograms based on the use of a phase-shifting technique. It can then use it to measure the surface and thickness of a parallel transparent object. However, phase errors occur owing to the inherent changes of the power of the tunable laser diode induced upon laser wavelength changes. To improve the measurement accuracy, we propose an optical power, real-time feedback control system, and a synchronous calibration scheme. The significance of the proposed method relies on the fact that the laser intensity variation can be detected quickly and compensated by adjusting the bias voltage. To verify the effectiveness of the proposed system, we incorporated it into the optical system of the wavelength-tuned, phase-shifting interferometer for experimentation. The control principles and the experimental validation of the proposed system are described in this letter.

Ultrasensitive ultrasound detection using an intracavity phase-shifted fiber Bragg grating in a self-injection-locked diode laser.

We report a high-sensitivity fiber-optic ultrasonic sensor system using a self-injection-locked distributed-feedback (DFB) diode laser where a π-phase-shifted fiber Bragg grating (πFBG) serves as both the locking resonator and the sensing element in a fiber ring feedback loop. By controlling the delay time of the feedback light through a fiber stretcher, the laser wavelength is locked to an external cavity mode on the spectral slope of the πFBG, and the ultrasound-induced wavelength shifts of the πFBG are converted to laser intensity variation. The ultrasonic sensing scheme simplifies the feedback control because the self-injection locking automatically pulls the laser wavelength to the πFBG resonant wavelength. In addition, it improves the detection sensitivity because the frequency noise of the DFB laser is drastically reduced. We show that the sensor system achieves a strain sensitivity of 78 fε/Hz1/2 at around 200kHz.

A detailed study of the quantum coherent and saturating resonances using the hyperfine lines of rubidium

We have studied both experimentally and theoretically the V-type system in the hyperfine structure of the D1 and D2 lines of rubidium with co-propagating pump-probe laser fields. The experimental observation in the V-type system shows the high contrast EIT peak along with the saturation peaks in the probe Doppler profile. A velocity dependent analysis regarding the formations and frequency positions of the observed resonances has been done. The dependence of the EIT line shape on the variation of the pump laser intensity and detuning has been investigated. It is observed that the EIT resonance disappears for a pump power when it is less than the probe power and the saturating effects dominate over the coherence effects in the system. By properly tuning the pump laser, a perfectly symmetric EIT peak with high contrast is obtained at the center of the probe Doppler profile, while the strong saturating resonances get eliminated. A theoretical model based on the density matrix formulation has been presented corresponding to a five-level V-configuration. This model helps us to interpret the experimental observation theoretically.

Photothermal lens technique: a comparison between conventional and self-mixing schemes

This work focuses on assessing the analytical capabilities of a new photothermal lens method based on the self-mixing effect to reliably measure metallic traces in water-ethanol solutions. We compare it with the conventional thermal lens scheme, considering the low detection limit and versatility. A theoretical model is presented to describe the laser power variations as a function of the photothermal parameters of the analyzed sample. The experimental results demonstrate that the laser intensity variations, induced by the external optical feedback, are governed by the photothermal lens effect. Measurements of Fe(II)-1,10-phenanthroline in water–ethanol solutions show a favourable correspondence and agreement with the theory. The low detection limits obtained by the two analytic techniques also agree very well. Nevertheless, our instrument presents advantages regarding compactness and simplicity, suggesting that this platform could be potentially useful as a robust analytical tool for metallic trace detection. In addition, calibration of the method is performed by measuring the so-called self-mixing constant.

Reduction of laser-intensity-correlated noise in high-harmonic generation.

We present a scheme for correcting the spectral fluctuations of high-harmonic radiation. We show that the fluctuations of the extreme-ultraviolet (XUV) spectral power density can be predicted solely by monitoring the generating laser pulses; this method is in contrast with traditional balanced detection used in optical spectroscopy, where a replica of the signal is monitored. Such possibility emerges from a detailed investigation of high-harmonic generation (HHG) noise. We find that in a wide parameter range of the HHG process, the XUV fluctuations are dominated by a spectral blueshift, which is correlated to the near-infrared (NIR) driving laser intensity variation. Numerical simulations support our findings and suggest that non-adiabatic blueshift is the main source of XUV fluctuations. A straightforward post-processing of the XUV spectra allows for noise reduction and improved precision of attosecond transient absorption experiments. The technique is readily transferable to attosecond transient reflectivity and potentially to attosecond photoelectron spectroscopy.

Creating Multifunctional Optofluidic Potential Wells for Nanoparticle Manipulation.

Optical forces have enabled various nanomanipulation in microfluidics such as optical trapping, sorting, and transporting of nanoparticles (NPs), but the manipulation is usually specific with a certain optical field. Tightly focused Gaussian beams can trap NPs but not sort them; moderately focused Gaussian beams allow sorting microparticles in a flow but not NPs; quasi-Bessel beams can sort NPs in a flow but cannot control their positions due to low trapping stiffness. All these methods rely on the axial variation of laser intensity. Here we show that multifunctional and tunable optofluidic potential wells can be created for nanomanipulation by synchronizing optical phase gradient force with fluiddrag force. We demonstrate controlled trapping and transporting of 150 nm Ag NPs over 10 μm and sorting of 80 and 100 nm Au NPs using optical line traps with tunable phase gradients in experiments. Our simulations further predict that simultaneous sorting and trapping of sub-50 nm Au NPs can be achieved with a sorting resolution of 1 nm using optimized optical fields. Our method provides great freedom and flexibility for nanomanipulation in optofluidics with potential applications in nanophotonics and biomedicine.

Quantifying Photothermal and Hot Charge Carrier Effects in Plasmon-Driven Nanoparticle Syntheses.

The excitation of localized surface plasmon resonances in Au and Ag colloids can be used to drive the synthesis of complex nanostructures, such as anisotropic prisms, bipyramids, and core@shell nanoparticles. Yet, after two decades of research, it is challenging to paint a complete picture of the mechanisms driving such light-induced chemical transformations. In particular, whereas the injection of hot charge carriers from the metal nanoparticles is usually proposed as the dominant mechanism, the contribution of plasmon-induced heating can often not be neglected. Here, we tackle this uncertainty and quantify the contribution of different activation mechanisms using a temperature-sensitive synthesis of Au@Ag core@shell nanoparticles. We compare the rate of Ag shell growth in the dark at different temperatures with the one under plasmon excitation with varying laser intensities. Our controlled illumination geometry, coupled to numerical modeling of light propagation and heat diffusion in the reaction volume, allows us to quantify both localized and collective heating effects and determine their contribution to the total growth rate of the nanoparticles. We find that nonthermal effects can be dominant, and their relative contribution depends on the fraction of nanoparticle suspension under irradiation. Understanding the mechanism of plasmon-activated chemistry at the surface of metal nanoparticles is of paramount importance for a wide range of applications, from the rational design of novel light-assisted nanoparticle syntheses to the development of plasmonic nanostructures for catalytic and therapeutic purposes.

Laser Self-Mixing Fiber Bragg Grating Sensor for Acoustic Emission Measurement.

Fiber Bragg grating (FBG) is considered a good candidate for acoustic emission (AE) measurement. The sensing and measurement in traditional FBG-based AE systems are based on the variation in laser intensity induced by the Bragg wavelength shift. This paper presents a sensing system by combining self-mixing interference (SMI) in a laser diode and FBG for AE measurement, aiming to form a new compact and cost-effective sensing system. The measurement model of the overall system was derived. The performance of the presented system was investigated from both aspects of theory and experiment. The results show that the proposed system is able to measure AE events with high resolution and over a wide dynamic frequency range.

Laser Intensity Variation in Amplitude and Phase Induced by Elliptically Polarized Feedback**Supported by the Program for Changjiang Scholars and Innovative Research Team in University under Grant No IRT160R7.

The laser output characteristics under elliptically polarized optical feedback effect are studied. Elliptically polarized light is generated by wave plate placed in the feedback cavity. By analyzing the amplitude and phase of the laser output in the orthogonal direction, some new phenomena are firstly discovered and explained theoretically. Elliptically polarized feedback light is amplified in the gain medium in the resonator, and the direction perpendicular to the original polarization direction is easiest to oscillate. The laser intensity variation in amplitude and phase are related to the amplified mode and the anisotropy of external cavity. The theoretical analysis and experimental results agree well. Because the output characteristic of the laser has a relationship with the anisotropy of the external cavity, the phenomenon also provides a method for measuring birefringence.

Variation of the band structure in DKDP crystal excited by intense sub-picosecond laser pulses

The nonlinear absorption (NLA) properties of potassium dideuterium phosphate crystals at 515 nm under different excitation laser intensities are investigated with the Z-scan technique. Two critical intensities are highlighted: the critical intensity for exciting the NLA and the critical intensity of the multiphoton absorption mechanism transition. Experimental results indicate the existence of defect states located in the band gap, which can be manipulated by varying laser intensity. A model based on the change of multiphoton absorption mechanism induced by the transformation of defect species is proposed to interpret the experiments. Modeling results are in good agreement with the experiment data.

Magnetometry using fluorescence of sodium vapor.

Magnetic resonance of sodium fluorescence is studied with varying laser intensity, duty cycle, and field strength. A magnetometer based on a sodium vapor cell filled with He buffer gas is demonstrated, using a single amplitude-modulated laser beam. With a 589nm laser tuned at the D1 or D2 line, the magnetic field is inferred from the variation of fluorescence. A magnetic field sensitivity of 150 pT/Hz is achieved at the D1 line. The work is an important step toward sensitive remote magnetometry with mesospheric sodium.

Intensity-independent wavelength locking of diode lasers to a spectral slope of a fiber-optic sensor for ultrasonic detection.

We propose and demonstrate a simple and low-cost method to lock the wavelength of a diode laser to a point with a particular normalized slope on the spectrum of a fiber-optic sensor. The wavelength locking point is independent of the laser intensity variations. The locking involves simultaneously and independently modulating both the laser wavelength and the laser intensity. On the spectral slope, the wavelength modulation is also converted to intensity modulation that is superimposed to the direct intensity modulation. The error signal is the amplitude of the overall intensity modulation. We demonstrate a potential application of the locking method in a fiber-optic ultrasonic detection system using a distributed feedback diode laser and a phase-shifted fiber Bragg grating sensor.

1,6-Diphenyl-1,3,5-hexatriene (DPH) as a Novel Matrix for MALDI MS Imaging of Fatty Acids, Phospholipids, and Sulfatides in Brain Tissues.

1,6-Diphenyl-1,3,5-hexatriene (DPH) is a commonly used fluorescence probe for studying cell membrane-lipids due to its affinity toward the acyl chains in the phospholipid bilayers. In this work, we investigated its use in matrix-assisted laser desorption/ionization (MALDI) as a new matrix for mass spectrometry imaging (MSI) of mouse and rat brain tissue. DPH exhibits very minimal matrix-induced background signals for the analysis of small molecules (below m/z of 1000). In the negative ion mode, DPH permits the highly sensitive detection of small fatty acids (m/z 200-350) as well as a variety of large lipids up to m/z of 1000, including lyso-phospholipid, phosphatidic acid (PA), phosphoethanolamine (PE), phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidylinositol (PI), and sulfatides (ST). The analytes were mostly detected as the deprotonated ion [M - H]-. Our results also demonstrate that sublimated DPH is stable for at least 24 h under the vacuum of our MALDI mass spectrometer. The ability to apply DPH via sublimation coupled with its low volatility allows us to perform tissue imaging of the above analytes at high spatial resolution. The degree of lipid fragmentation was determined experimentally at varying laser intensities. The results illustrated that the use of relatively low laser energy is important to minimize the artificially generated fatty acid signals. On the other hand, the lipid fragmentation obtained at higher laser energies provided tandem MS information useful for lipid structure elucidation.

Comprehensive CO detection in flames using femtosecond two-photon laser-induced fluorescence

We demonstrate a femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique for sensitive CO detection, using a 230 nm pulse of 9 µJ and 45 fs. The advantages of fs-TPLIF in excitation of molecular species were analyzed. Spectra of CO fs-TPLIF were recorded in stable laminar flames spatially resolved across the flame front. A hot band (1, n) together with the conventional band (0, n) of the B→A transitions were observed in the burned zone and attributed to the broadband nature of the fs excitation. The CO fs-TPLIF signal recorded across the focal point of the excitation beam shows a relatively flat intensity distribution despite of the steep laser intensity variation, which is beneficial for CO imaging in contrast to nanosecond and picosecond TPLIF. This phenomenon can be explained by photoionization, which over the short pulse duration dominates the population depletion of the excited B state due to the high peak power, but only contributes in total a negligible X state depletion due to the low pulse energy. Single-shot CO fs-TPLIF images in methane/air flames were recorded by imaging the broadband fluorescence. The results indicate that fs-TPLIF is a promising tool for CO imaging in flames.

Polarization smoothing for single beam by a nematic liquid crystal scrambler.

Polarization smoothing (PS) is a key approach to suppress laser plasma instabilities (LPI) in inertial confinement fusion (ICF) experiments. Here, we propose a liquid crystal (LC) PS element to realize single beam smoothing and demonstrate its smoothing effect, in principle, with a 2×2 LC polarization checkerboard, which reduces the laser intensity variation in the focal spot to 78.4%. LC PS elements, which have potential applications in high-power ICF laser drivers, have many advantages because they are easy to fabricate, cost effective, flexible, and large.